The present invention refers to a final bridge stage for a receiver audio amplifier which operates on low voltage and is monolithically integratable and useable in both consumer telephones and electromedical devices for persons having poor hearing, in order to control an electroacoustic transducer.
The final stage of a receiver amplifier for a consumer telephone must be able to effect a rigorously constant absorption of current with respect to a variable input signal. In fact, the terminals of a bifilar telephone line to which the telephone is connected are at the same time its feeder terminals, as well as the output terminals for the signal that the telephone must transduce. Therefore, variations in the absorption of the receiver amplifier current could be erroneously interpreted as signal variations.
Moreover, the final stage of a receiver amplifier connected to a low voltage bifilar telephone line must have a low current absorption and a low "voltage loss", defined as the difference, in normal operating conditions, between the voltage to the line terminals and the voltage resulting from the final stage to which the electroacoustic transducer is connected.
A smaller limitation of the dynamic `range` of the amplified signal corresponds to a smaller loss of current of the final stage.
In telephony however, it is important to try to minimize the absorption of current, even if so doing results in a slightly larger voltage loss. The final stages for receiver audio amplifiers which offer the best dynamic `range` of the signal at a current absorption equivalence, are those which have a "bridge" circuit.
In present technology, a final bridge stage for receiver audio amplifiers, which is monolithically integratable and useable, for example, in telephony, consists, as is illustrated in FIG. 1, of a bridge configuration, comprising a pair of constant current generators, indicated as A1 and A3, and a pair of bipolar transistors (NPN type), T2 and T4.
The T2 collector and the T4 collector to which, respectively, are connected the first and second terminals of a TR electroacoustic transducer, at point A and point B respectively, are in turn connected to the positive pole, +VCC, of a supply voltage generator, through generator A1 and generator A3 respectively. The T2 and T4 emitters are connected to the negative pole, -VCC, of the supply voltage generator.
The bases of T2 and T4 are connected to the emitters of two bipolar transistors (NPN type), T12 and T14 respectively, which control T2 and T4. They are also connected to the negative pole, -VCC, of the supply voltage generator through a resistor R2 and a resistor R4 respectively. The T12 collector and the T14 collector are connected to +VCC; the bases of T12 and T14 are connected to a signal source (which is not illustrated), and which controls such transistors in phase opposition.
The current absorption of a final bridge stage of this type is constant and equal to the sum of the currents produced by the A1 and A3 generators, even in the absence of a signal.
Typically, the A1 and A3 current generators consist of PNP bipolar transistors which function in the active zone. Their emitters are connected to +VCC, and their collectors are connected to the T2 and T4 collectors respectively. The total loss of voltage of the prior art stage is therefore equal to the sum of the collector-emitter voltage of a transistor, T2 or T4, (NPN type), in saturation (VCE sat.), and of the collector-emitter voltage of a transistor (PNP type), A1 or A3, in saturation (VCE sat.), at the limit of the active zone.